1. Field of the Invention
The present invention relates generally to the operation of hybrid vehicles, and more particularly to the operation of a battery system of a hybrid electric vehicle.
2. Background Art
Hybrid electric vehicles use an internal combustion engine and a battery as power sources for vehicle propulsion. In general, the battery performs poorly at cold temperatures. Specifically, cold temperatures affect battery chemistry and increase electrical resistance within the battery. As a result, the peak charge of the battery is limited and the peak battery discharge capability is reduced. Poor battery performance reduces torque to the vehicle wheels when battery power is utilized to propel the vehicle and increases reliance on the internal combustion engine. Greater reliance on the engine increases vehicle emissions and decreases fuel economy. In addition, poor cold temperature battery performance could result in the need to add external or secondary battery heating devices to the hybrid electric vehicle.
Cold temperatures also increase the effects of polarization resistance voltage of the battery, which at sufficiently high levels can prevent the battery from charging or discharging. Polarization resistance voltage can be temporarily lowered by reversing the polarity of the battery, such as by switching from discharging the battery to charging, or vice versa. Reducing the polarization resistance voltage allows more current to circulate through the battery. The current circulating through the battery interacts with the internal battery resistance (i.e., I2R loss), resulting in a battery temperature increase and improved battery performance at cold temperatures. However, battery current polarity reversals may be noticeable to vehicle occupants. For example, polarity reversals may be manifested as noise, intermittent engine revving, and hesitation in vehicle acceleration and may be particularly noticeable when cruising at a steady speed.
Consequently, there was a need for a method of heating a hybrid electric vehicle battery to improve battery performance and to do so in a manner that is not objectionable to vehicle occupants. Problems associated with prior art as noted above and other problems are addressed by Applicant's invention as summarized below.